This invention relates to a selective hydrogenation catalyst for the selective hydrogenation of unsaturated hydrocarbons, a process for preparing this catalyst and its use, more specifically, to the selective hydrogenation catalyst for selectively hydrogenating acetylenic and diolefinic compounds having two to four carbon atoms into corresponding olefin and its use in petroleum hydrocarbon thermal cracking process.
It is well known that the steam cracking of petroleum hydrocarbon is the most important process for manufacturing the monomers of polyolefin such as ethylene, propylene. Its process route is that after mixing petroleum hydrocarbon with steam, the mixture stream in cracking furnace is thermal cracked at 750-870xc2x0 C. to form H2, CO, CO2 and the mixture of saturated and unsaturated aliphatic hydrocarbons having one to three carbon atoms and aromatic hydrocarbons. By passing through a series of fractionating towers, the feed stream is fractionated into fractions of  greater than C10, H2 and CH4, C2, C3, C4, C5-C10. In C2-C10 fractions, there exist unsaturated hydrocarbons such as mono-olefin, alkyne, diolefin etc. The alkyne and diolefin in C2, C3 fractions are the poisonous impurities which interfere the follow up polymerizations of ethylene, and propylene. They make the activity of polymerization catalyst decrease and catalyst consumption increase, moreover the performance of polymerization product poor. It is needed to control strictly the amount of diolefin and alkyne such as acetylene contained in the cracked product. In recent years, the high efficiency catalyst for the polymerization of ethylene newly developed gets more and more strict on the concentration requirement of alkyne and polyolefin. For example, for C2 fraction, the acetylene content was limited to 10xe2x88x926 mol % or less after 1980""s, and for some process having special requirement such as the manufacture of HDPE, the acetylene content in refined ethylene was limited to 0.1xc3x9710xe2x88x926 mol % or less.
Therefore, how to remove more efficiently the highly unsaturated hydrocarbon such as acetylene from the cracked product has been concerned generally. In general, the catalytically selective hydrogenation method is adopted to remove the acetylene in ethylene-ethane fraction. In the reaction of selective hydrogenation for acetylene removal, the whole acetylene should be converted while no over-hydrogenation occurs such that ethylene loss is avoided. Consequently, both the superior activity and higher selectivity of the hydrogenation catalyst are required. Moreover, the acetylene absorbed on catalyst surface is easily dimerized through hydrogenation to form unsaturated C4 hydrocarbons such as 1,3-butadiene, and such a C4 hydrocarbon would react subsequently with acetylene or ethylene or other unsaturated hydrocarbon to form C6-C24 oligomers (known as xe2x80x98green oilxe2x80x99). A part of so-called green oil, flowing along with the feed stream, is removed in a green oil absorber, and the other part would adhere on the catalyst surface and cover on the active center of catalyst, which will make its hydrogenation activity and selectivity decrease gradually, shorten the catalyst operation cycle period and lead to more frequent regeneration, affect the service life, thus increase in production costs. Therefore, the selective hydrogenation catalyst having superior performance should have high activity, selectivity and lower amount of green oil formation, particularly the amount of green oil adhered on the catalyst should be low.
In the prior art, generally a large quantity of supported Pd catalyst is adopted and other cocatalyst component added. For example, cocatalyst component disclosed in U.S. Pat. No. 4,404,124 is Ag, that in EP892,252 is Au, that in DE 1,284,403 and U.S. Pat. No. 4,577,047 is Cr, that in U.S. Pat. No. 3,912,789 is Cu, that in U.S. Pat. No. 3,900,526 is Fe, that in U.S. Pat. No. 3,489,809 is Rh, that in U.S. Pat. No. 3,325,556 is Li, that in CN1151908A is K; moreover other cocatalyst components disclosed include Pb, Zn etc.
In above-mentioned cocatalyst components disclosed, some of them sacrificed Pd catalyst""s selectivity due to improving its activity, some of them can improve the Pd catalyst""s selectivity or reduce the green oil formation, but their activity reduced significantly. Consequently, only the Pdxe2x80x94Ag catalyst is in common use industrially at present. But with respect to its activity and selectivity, Pdxe2x80x94Ag catalyst is still less than satisfactory, particularly the control of green oil formation on catalyst surface is undesirable yet.
In connection with the above disadvantages, a selective hydrogenation catalyst for alkyne is disclosed in the Chinese Patent Application CN 1,279,126A, wherein the main catalyst component is Pd and cocatalyst component is Bi and Ag. When the catalyst system is used in the selective hydrogenation of alkyne, it reduces green oil formation obviously while exhibits higher activity and selectivity, thus the carbon deposit decrease and service life of the catalyst increase.
However, said catalyst is still less than satisfactory. Along with the increasing of alkyne handling capacity in industrial application and the aggravation of side reaction at severer reaction conditions such as high space velocity (7,000-20,000 hrxe2x88x921) and high alkyne content, the activity, selectivity, the regeneration period and service life of said catalyst are unsatisfactory, improvements thereof are still needed. In general, the catalytic ability of the catalyst decreases due to deposition of carbon on the catalyst after a long time of alkyne selective hydrogenation, and the catalyst should be regenerated by heating to about 500xc2x0 C. with air blow so as to recover the activity and selectivity if it fail to meet the production requirement even at an elevated temperature. The temperature, at which a catalyst is regenerated, must be controlled strictly. But even so, the technical parameters of the catalyst such as specific surface area and the like are also changed greatly after enduring 3-5 times of regeneration. The poor repeatability and stability of catalyst will result in the deterioration of its properties until it can not be used again and must be substituted with a new catalyst.
It is known that rare earth metals can be used in hydrogenation catalyst wherein Pd acts as main active component. For example, in U.S. Pat. No. 5,426,253, a process for hydrogenating 2,2-dichloro-1,1,1,2-tetrafluoroethane and 2-chloro-1,1,1,2-tetrafluoroethane or a mixture thereof into 1,1,1,2-tetrafluoroethane is disclosed, wherein the main active component of the adopted catalyst is a metal of Group VIII selected from the group consisting of Pt, Pd, Ni, Rh, Co. Ru and Ir. Moreover, said catalyst contains at least one of the following metals: Cu, Ag, Au, La, Ce and Nd, and which is disclosed to be used as the cocatalyst with a corrosion-resistance in the hydrogenation and dechlorination of halohydrocarbon concerned in this patent so as to protect the activity of the catalyst from the influence of HCl formed in the dechlorination.
As a result of making repeated experiments, the present inventors discover that by means of the supporter loaded concurrently main active component Pd, cocatalyst component Bi, rare earth metals and at least one of Ag, Cu, K, Na, Sr, Mn, Zr, Mo, Co, Ge, or a combination of two or more metals thereof, the hydrogenation reaction of acetylene is carried out in higher selectivity and higher activity under a high space velocity, wherein said acetylene exists in the C2 fraction resulted from petroleum hydrocarbon thermal cracking process. Moreover, the catalyst of the present invention has the advantage of good stability, small changes in catalyst""s properties after multiple times regenerations, easily controllable physical parameters, improved regeneration period and service life due to the addition of rare earth metals.
The object of this invention is to provide a selective hydrogenation catalyst for selectively hydrogenating unsaturated olefin.
Another object of this invention is to provide a process for preparing this catalyst.
A further object of this invention is to provide a use of said catalyst in the petroleum hydrocarbon thermal cracking process for selectively hydrogenating C2 fraction or C3 fraction including high-unsaturated hydrocarbons such as acetylene etc. into olefin.
A selective hydrogenation catalyst for selectively hydrogenating unsaturated hydrocarbon of the present invention, which comprises mainly the following active components loaded on a porous inorganic supporter:
(1) at least one of Pt, Pd, Ni, Ru, Co, and Rh;
(2) at least one of Ag, Cu, Zn, K, Na, Mg, Ca, Be, Sn, Pb, Sr, Ba, Ra, Fe, Mn, Zr, Mo, and Ge;
(3) at least one of rare earth metals; and
(4) Bi;
wherein the rare earth metal is selected from Sc, Y, and Lanthanides in Group IIIB of periodic table of elements.
The preparation method of the catalyst of the present invention is: the rare earth metal component being loaded first, or the rare earth metal component and K or Na component being loaded concurrently first, and then other active components being loaded stepwise or concurrently.